Whilst I despise the cultish messianic culture that has developed around Greta (seriously, some of the fanaticism around her is frightening and akin to a religious orthodoxy), and find her rather irritating (albeit well-intended, as you say), she has my full respect for such snarky comebacks to these foolish individuals. If they find her annoying, they should simply do what I do to anyone or anything that annoys me: Ignore with extreme prejudice.